(a) Field of the Invention
The present invention relates to a colored styrenic resin composition which is useful as a material for the production of housings of, for example, a variety of electric appliances and office machinery and equipment as well as miscellaneous goods.
(b) Description of the Prior Art
Styrenic resins have widely been used as materials for forming various products because of the well-balanced processing properties, dimensional stability and mechanical strength thereof. In particular, they have been used for making housings of, for example, a variety of electric appliances and office machinery and equipment as well as external parts for miscellaneous goods. They are in general used in colored or pigmented conditions from the viewpoint of design. In the coloration of styrene resins used in these applications, titanium oxide is often used and the coloration thereof is usually carried out by kneading the resin with a mixture comprising titanium oxide and a dispersant such as a metal salt of a fatty acid for uniformly dispersing the titanium oxide in the resin. When styrenic resins colored with titanium oxide are injection-molded, however, the injection-molded product sometimes has a stripe-like pattern even though black pigment is not used. This black stripe-like pattern is in general called a "black streak".
The formation of the foregoing black streaks greatly impairs the commercial value of the product. For this reason, many attempts to eliminate this problem have been made. For instance, Japanese Unexamined Patent Publication (hereinafter referred to as "J.P. KOKAI") No. Sho 63-86743 discloses a method for solving the problem which comprises adding, to a styrenic resin composition containing titanium oxide and a metal salt of a fatty acid, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, distearylpentaerythritol diphosphite, tetrakis(2,4-di-t-butylphenyl)-4,4'-biphenylene diphosphite or boron trioxide and J.P. KOKAI No. Hei 2-88659 discloses the use of alkylene-bis-fatty acid amide or a zinc salt of a fatty acid as a dispersant for the coloring agent. However, both of these methods are insufficient for completely preventing the formation of black streaks.
The cause for the formation of black streaks has not yet been clearly elucidated, but it would be assumed that the problem of the formation of black streaks is caused by titanium oxide per se since the black streaks are always formed when the resin composition comprises titanium oxide as a coloring agent irrespective of the presence of a dispersant. In other words, it would be recognized that the black streaks are formed for the following reason. The titanium oxide is dispersed in a molten resin in the form of particles, the titanium oxide particles are plated-out from the molten resin during the injection molding thereof and the particles scrape off the surface of the cylinder and/or screw of the molding machine.